Trains accelerate incredibly slowly and often have a single gear that is optimised for high speed. Similar to trying to pull away in your car in too high of a gear this makes trains really slow and takes along time for them to hit their high speed.
Most cars will often have 4 (for very old cars) – 6 gears to keep acceleration smooth and fast whilst still being efficient at high speed but trains don’t.
I get that electric motors have all of the torque available at low RPMs whilst ICE only have full power at a high rpm but wouldn’t gears still allow trains to get to higher speeds quicker?
In: Engineering
Trains are amazingly underpowered compared to cars. Freight trains are somewhere around 0.5 to 2 horsepower per ton. You’re average car is somewhere around 100 horsepower per ton. That’s why trains can’t go up hills more than about a 2% grade and accelerate very slowly.
However, they are very efficient. Trains can move a ton of freight over 470 miles on a single gallon of fuel.
Trains are awesome in that they **only need one gear**. Train tracks have much stricter limits for grade and turn radius than roads.
Since people aren’t strapped into a train like they are in a car and may not even be sitting down, having trains accelerate very fast would be quite uncomfortable or even dangerous for the people inside.
Electric cars don’t have gears, too. Electric vehicles don’t need them.
Trains accelerate slowly because of steel tires on stelle tracks. There is not much grip.
This is going to be a very basic explanation, I understand I’m leaving out a lot of important details here.
Trains dont actually have any gears. A modern diesel electric train is essentially a generator.
Inside the locomotive, a diesel engine is coupled to a gear box. Let’s say 1:25 ratio (just an example). So for every 1 revolution of the crank (output) shaft of the diesel engine, there are 25 revolutions on the output shaft of the gearbox.
The output of the gearbox is then coupled to a generator. Electric generators and motors are essentially the same thing. Apply mechanical movement to a generator/motor and you will produce electricity. Apply electricity to a motor/generator and you will produce mechanical movement.
The power that is generated from the locomotive is used to power everything on the train. Lights, controls, heating, air compressors, and the motors that move the train.
Those motors are located under the locomotive, between each set of wheels, much the same way electric cars work.
The electricity that was generated in the locomotive is fed to the motors through a variable frequency drive (VFD). The VFD controls motor output speed by changing the frequency of the sine wave (Dont worry about this part). It kind of limits the amount of power applied to the motors.
Those motors are then connected to each set of 2 wheels through another gearbox, this time stepping down, so let’s say 25:1 ratio (again just an example). So for every 25 revolutions of the motor (input of the gearbox), there is 1 revolution of the wheel (output).
Trains are slow because they produce an insane amount of torque, and the process I outlined is what makes that torque. As others have mentioned, slow speed, high torque is exactly what trains need, as they are very heavy, and have little friction between the wheels and the rails to work with.
As they start to gain momentum, the frequency can be turned up on the VFDs to slowly add more speed, and less torque to the wheels, this is essentially what the throttle control on a locomotive does. (Again, very simplified here).
Another benefit of this is the ability for trains to use regenerative braking, the same way electric cars do. (I wont get into this),
Streetcars, subways, LRT vehicles, and some passenger/freight trains use the same process of adjusting frequency to drive motors, but instead of generating the electricity right inside the locomotive, power is fed to the train by an overhead wire, or a third rail.
The “torque from the start” isn’t universally true for all types of electric motors, and it’s also not entirely accurate. Many types of electric motors actually have a lot more torque available at a standstill, while others only reach their maximum torque once they spin up. Ceiling fans for example have barely any torque from a standstill, which is why they take so long to spin up and why you can easily stall them.
However, you are not wrong: For maximum acceleration, electric motors can benefit from a gearbox in order to keep the motor running in its optimum RPM range. Porsche does that with their new electric sports car. However, this adds weight and complexity, and electric trains usually have plenty of acceleration anyways.
The problem is not the available torque in the engine. But there is a few issues with trains that make their initial departure very slow compared to cars. The first issue is that the train is very long. When the locomotive starts moving the back of the train does not immediatly start moving. As the locomotive moves forward it will start taking up the slack between the cars and also stretch out the frames of the cars a bit. Even a very tight passenger train will have a few seconds from the locomotive starts moving to the back cars starts moving. And first then will the momentum of the back cars make it lighter for the locomotive. This causes a jerking motion going back and forth through the train as it gets up to speed. In order to make this jerking motion much less which increases comfort and reduces wear on the equipment the engineers will start with the minimum of throttle and slowly increase it as they get up to speed. The second issue is that the train tracks and wheels have very low friction. This is very good for high loads and high speeds but terrible for acceleration and braking. If the engineer applies full throttle from a standstill the wheels will just slip on the smooth tracks and he will spin in place damaging both the tracks and wheels. So he have to get up to speed where he have some momentum before he can apply the full power.
Some trains do actually have gears. Diesel multiple units (DMUs) are often used in the UK and other countries for short passenger runs. They consist of 1-3 train cars, with diesel engines underneath. The engines are coupled to the wheels either via a mechanical gearbox (usually with automatically changing gears), a hydraulic transmission (potentially also with a gearbox), or electrically via a generator and motor.
The mechanical gearboxes are kinda rare nowadays, but there are still some trains (class 143/144 Pacers) in the UK that use them – albeit that they’re on schedule to be replaced pretty soon.
Also, smaller lighter trains can have better acceleration – underground trains in particular are pretty snappy. They’re never going to be going all that fast, so I guess the motors can be optimised for high acceleration.
it has less to do with not having the power and more about respectin g the limitations of the equipement so that they last longer and are more confortable for the cargo.
when you start moving on a train, only the locomotive has any actual propulsion that is used ot start pulling on othe other cars as it pick up the slack. this causes some stress and the frames ot expand slightly causing a bakc and forth motion. this can be minizined if the conductor slowly increases/decreases speed which in turn makes htis motion less noticeable and causes less wear and tear on the equipement.
It’s not a gear actually in the strictest sense. It does have teeth like a gear. It’s a traction motor. The electric motor turns the single gear and the teeth spin the wheel. It’s the same reason an electric car has no transmission.
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